1. Field of the Invention
This invention relates to a fuel cell apparatus, more particular, it relates to the fuel cell apparatus in which a discharge gas having a pressure energy even after an oxygen has been consumed at the fuel cell is expanded at an expanding machine (expander) to collect the pressure energy in the discharge gas as a mechanical energy for assisting driving of a compressor.
2. Related Art
In this kind of fuel cell apparatus, the fuel cell is provided with an air supply tube extending from a compressed air source to the fuel cell, and a discharge gas tube extending from the fuel cell to an atmosphere; the compressor is disposed on the air supply tube, and the expander is disposed on the gas discharge tube respectively. The compressor is driven by an electric motor.
For example, a fuel cell apparatus disclosed in Japanese Unexamined (koukai) Patent Application No. 7-14599 which corresponds to U.S. Pat. Nos. 5,434,016 and 5,645,950, as shown in FIG. 3, includes a fuel cell 50 provided with an air supply tube 51 and a gas discharge tube 52, a compressor 53 disposed on the air supply tube 51, an expander 54 disposed on the gas discharge tube 52, and an electric motor 56 for driving the compressor 53. The air supplied from the air supply tube 51 is compressed to a predetermined pressure value by the compressor 53 and then is supplied to the fuel cell 50, to consume oxygen contained therein at the fuel cell 50. The discharge gas of which oxygen is consumed at the fuel cell 50 is then discharged through the gas discharge tube 52, expanded by the expander 54 down near to an atmospheric pressure. The expander 54 is disposed coaxially with the compressor 53 and is connected thereto by a common shaft 57 to be driven by the electric motor 56. In FIG. 3, reference numerals 58 and 59 each show a water separator, 61 shows a pump, 62 shows a nozzle, and 63 shows a tank, respectively.
This fuel cell apparatus intends to utilize the pressure energy remaining in the discharge gas discharged from the fuel cell 50 for assisting driving or power of the compressor 53. That is, the pressure energy of the discharge gas is converted to mechanical energy (driving force) by the expander 54, and the driving force is transmitted via the common shaft 57 to the compressor 53 to assist driving of the compressor 53 by the electric motor 56.
However, the assisting drive force for the compressor 53 by the expander 54 can be obtained only after a discharge pressure in the gas discharge tube 52 increases up to a predetermined value (rating absorb pressure of the expander 54), which is realized with a lapse of some time after an operation start of the fuel cell. In other words, before the discharge pressure in the gas discharge tube 52 reaches the predetermined value such as just after start of the fuel cell, expander 54 can not be operated by the discharge gas, so it is operated by the electric motor 56 to expand the discharge gas compulsorily down to below the atmospheric pressure. This means however not only the expander 54 can not collect mechanical energy but the expander 54 consumes electric energy of the electric motor 56, in other words, a negative torque is transmitted from the expander 54 to the electric motor 56.
In view of the above, in this conventional fuel cell apparatus, a swing plate valve or an air register 64 is provided between the fuel cell 50 and the expander 54 on the gas discharge tube (upstream of the expander 54), to introduce the atmosphere into the gas discharge tube 52, when the pressure in the gas discharge tube 52 is negative, for increasing the pressure therein.
However, even after the swing plate valve 64 is opened, the rotation of the expander 54 by the electric motor 56 continues until the pressure in the gas discharge tube 52 reaches the predetermined value, and the electric energy of the electric motor 56 is consumed for driving the expander 54. In addition, due to the atmosphere introduced through the swing plate valve 64 by the expander 54, amount of the air in the gas discharge tube 52 and the expander 54 increases, so that necessary time for increasing the pressure of the discharge gas in the discharge gas 52 is meaninglessly extended.